Improving Swift-XRT positions of GRBsP. A. EvansA. P. BeardmoreM. R. GoadJ. P. OsborneD. N. BurrowsN. Gehrels2381/39076https://leicester.figshare.com/articles/Improving_Swift-XRT_positions_of_GRBs/10201541Since GRBs fade rapidly, it is important to publish accurate, precise positions at early
times. For Swift-detected bursts, the best promptly available position is most commonly the X-ray
Telescope (XRT) position. We present two processes, developed by the Swift team at Leicester,
which are now routinely used to improve the precision and accuracy of the XRT positions reported
by the Swift team. Both methods, which are fully automated, make use of a PSF-fitting approach
which accounts for the bad columns on the CCD. The first method yields positions with 90% error
radii <4.4" 90% of the time, within 10–20 minutes of the trigger. The second method astrometrically
corrects the position using UVOT field stars and the known mapping between the XRT and UVOT
detectors, yielding enhanced positions with 90% error radii of <2.8" 90% of the time, usually 2
hours after the trigger2017-01-09 09:47:10Science & TechnologyPhysical SciencesTechnologyAstronomy & AstrophysicsInstruments & InstrumentationSwiftgamma ray burstsastrometryTELESCOPE